I do understand that, but I think you are applying a post hoc rationalisation to the change.
For example, examining the change through the lens of intended use -> you can't as there is no such use of the field today - it's tomorrow's use that is potentially problematic.
I don't want to wait until a bad actor applies the field, I want to stop the field from existing.
This change is not happening in isolation. There is currently a general trend towards de-anonymising users, and this DOB field is a step in that direction.
The only real question is, do I want my computer storing more, or less, personally identifying information. Given that I don't trust ANY use which may be later enabled by this change, my answer is 'less'.